284 THE EVOLUTION THEORY 



that every isolation of a species-colony conditions its transmutation 

 to a new species, or, as has been maintained, first by Moritz Wagner, 

 and later by Giilick and by Dixon, that isolation is a necessary pre- 

 liminary to the variation of species — that not selection but isolation 

 alone renders the transmutation of a species possible, and thus admits 

 of its segregation into several different groups of forms. Romanes 

 went so far as to regard the natural selection of Darwin and Wallace 

 as a sub-species of isolation, and isolation in its diverse forms he 

 regarded as the sole factor in the formation of species. He assumed 

 that it was only b}^ the segregation of individuals which did not 

 vary that the constant reversion to the ancestral species could 

 be prevented, and he regarded the process of selection as essentially 

 resultino- in the ' isolation ' of the fittest throuo;h the elimination 

 of the less fit. The idea is correct in so far, that selection undoubtedly 

 aids the favourable variation to conquest over the old forms, precisely 

 because the latter, being less favourably placed in the struggle for 

 existence, are gradually more completely overcome and weeded out, 

 so that a constant mingling of the new forms with the old is prevented, 

 just as it is by isolation of locality. Obviously the new and fitter 

 forms could not become dominant, could not even become permanent, 

 if they were always being mingled again with the old. But whether 

 it serves an}^ useful purpose to bring this undei* the category of 

 ' isolation,' and to say that mingling with the ancestral form during 

 transmutation is prevented by natural selection, in that favourably 

 varying individuals are isolated by their superiority from the inferior 

 ones, that is, the non-var^^ing individuals which are doomed to 

 elimination, is somewhat doubtful. For my part, I should prefer 

 to retain the original meaning of the word, and to call ' isolation ' the 

 separation of a species-colony by spatial barriers. 



Whether this factor by itself prevents the mingling with the 

 ancestral form as effectually as selection does, and wdiether isolation 

 alone and by itself can lead to the evolution of new forms, or perhaps 

 inust lead to them, must now by investigated. 



I look at this question from exactly the same point of view as 

 I did nearly thirty years ago, when in a short paper ^ I endeavoured 

 to show that, under favourable circumstances, an individual variation 

 of a species may become the origin of a local variety if it finds itself 

 in an isolated region. Suppose an island had no diurnal butterflies, 

 vintil one day a fertilized female of a species from the continent was 

 driven thither, found suitable conditions of life, laid its eggs, and 

 became the founder of a colon}^ : the prevention of constant crossing 



* Ueber den Einfluss der Isolinmg aitf die Artbildung, Leipzig, 1872. 



